Several methods are known for making polarizing glasses. For example, it has been demonstrated that polarizing glasses can be produced from silver halide-containing glasses by a redraw process as disclosed in U.S. Pat. No. 3,540,793 (Araujo et al.), or by subjecting stretched glass to a reducing gas atmosphere as disclosed in U.S. Pat. Nos. 4,304,584 and 4,479,819 (Borrelli et al.). In the redraw process, glass containing a separable phase is stretched or redrawn above its softening temperature, during which process, the separable phase is elongated. The thermal treatment which leads to the phase separation is carried out before the redraw process. In one version of the above process, the separated phase is initially spectrally non-absorbing material such as AgClBr, CuClBr or copper/cadmium halides, which must be subsequently modified to create a desired dichroic property necessary for the polarizing effect. This is accomplished by treating the stretched glass in a reducing gas (e.g., hydrogen) environment at elevated temperatures for sufficient time to effect the chemical reduction of the spectrally non-absorbing materials to their corresponding metals. The chemical reduction process is a combined process involving both the diffusion of hydrogen in the glass and the chemical reaction of the hydrogen with the halide phase. The polarizing behavior derives from the reduced layer. Also, when the polarizing glass is heated to the vicinity of 500° C. for any prolonged period of time, the elongated particles respheroidize and the polarizing property is lost. That is, the elongated particle returns to its spherical shape. This is explained by the fact that once the glass is soft enough, the interfacial forces act to undo what the redrawing forces had accomplished.
Hydrogen firing at elevated temperatures to change the color of glasses whose compositions contain reducible ions is also well known. A notable commercial application of that technique is found in the Corning incorporated eyewear product lines marketed under the SERENGETI® and CPF® trademarks. The color changes induced are attributed to the reduction of a portion of the silver halide in the glass to form quasi-spherical silver particles. The chemical reaction proceeds very fast relative to the hydrogen diffusion which leads to the condition of a sharp boundary between the reduced region near the surface, and the unreduced region below the surface.
In addition to silver halide-containing glasses, it has also been shown that copper and cadmium halide photochromic glasses disclosed in U.S. Pat. No. 3,325,299 can also be rendered polarizing in the darkened state when thermally softened and stretched, or otherwise elongated. This action elongates the halide crystals and is described in detail in U.S. Pat. No. 3,954,485 (Seward III, et al.). U.S. Pat. No. 5,517,356 (Araujo et al.) discloses a glass polarizer having a precipitated crystal phase selected from cuprous, cadmium and mixed cuprous-cadmium halide.
For certain applications, particularly telecommunications applications, one disadvantage of manufacturing polarizing glass elements by the methods referenced above is that phase-separated silver halide phase remains in the central layer of the glass element after the outer layer has been reduced to elemental silver. This phase-separated silver halide phase contributes to scattering of light and decreased transmission through the polarizing element. It would be desirable to provide methods capable of producing polarizing glass elements that reduce or eliminate the presence of a phase-separated phase in the central layer of the glass. Thus, there continues to be a need for new and improved methods of forming polarizing glass elements.